Lens protection mechanism

ABSTRACT

A lens protecting mechanism ( 104 ), comprising: a transparent disc ( 122 ) having a rim ( 118 ); a plurality of wheels that engage and support the rim; and at least one motor, coupled to at least one of said wheels to apply rotational force to said disc ( 122 ).

RELATED APPLICATIONS

The present application is a U.S. national application ofPCT/IL00/00750, filed Nov. 15, 2000.

FIELD OF THE INVENTION

The present invention is related to the field of lens protectors andespecially centrifugal lens protectors.

BACKGROUND OF THE INVENTION

Cameras are required to operate under many adverse conditions, includingflying particles, such as rain and snow. Rain and snow may adhere to thecamera lens, reducing image quality.

One well known solution is to place a spinning glass disc before thelens, so that all the particles are captured by the disc. Thecentrifugal force generated by the disc as it rotates throws theparticles towards the circumference of the disc, where they fall offand/or otherwise stop interfering with the image.

U.S. Pat. No. 3,879,742 to Smith, the disclosure of which isincorporated herein by reference, has the disc mounted on a centralaxis, with a surveillance camera at an off-axis location relative to thedisc.

U.S. Pat. No. 5,223,880 to Rapp, the disclosure of which is incorporatedherein by reference, integrates the disc with a rotor of a magneticmotor, with a casing surrounding the disc being the motor housing.

U.S. Pat. No. 5,315,333 to Nash, the disclosure of which is incorporatedherein by reference, provides a single motor coupled to the disc using aband. The disc is mounted on two lines of bearings. The lens protectiondevice is mounted on the camera, to isolate lens from gyroscopic andvibration forces.

Australian application 9515074, the disclosure of which is incorporatedherein by reference, describes another band-based system, in which thedisc is said to rotate at between 2600 and 3000 RPM. It is suggested touse a curved or hydrophobic surface to assist in water removal. Themotor can be powered using the camera battery pack or using a separatebattery.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention relates to symmetricapplication of force to a rotating transparent disc. In some embodimentsof the invention, this symmetry reduces vibration, thus possiblyreducing wear, noise and/or image degradation. In other embodiments,asymmetric application of force and/or support is provided, possiblyalso achieving the desired results of reducing noise, wear and/orvibration.

In some embodiments of the invention, the rotational force is applieddirectly to the disc by one, two, three, four or more motors located atthe circumference of the disc. In some embodiments of the invention, aplurality of wheels are distributed around the disc, engaging and/orsupporting the disc. Each such wheel may be motor driven or freeturning. Optionally, but not necessarily, the wheels are evenlydistributed around the circumference of the disc.

An aspect of some embodiments of the invention relates to balancingangular moments in a centrifugal lens protection device. In someembodiments of the invention, by balancing angular moments, themagnitude of gyroscopic effects are reduced substantially. In someembodiments of the invention, the weight, radius and/or rotations speedof the disc and the motors and/or wheels used to rotate the disc arematched, such that a total angular moment is close to zero. Thus, forexample, the total weight of a wheel may be twice the weight of thedisc, to make up for a larger radius of the disc. In some embodiments ofthe invention, all the rotations are in a single plane. In someembodiments of the invention, the wheels are located at 45° to thehorizontal and vertical axis, to better provide support for gyroscopicforces that are usually at about 45° to the axis of motion. In a camera,the axis of motion is expected to be parallel or perpendicular to thegravitational field. Optionally, the wheels locations may be flattenedout to be more horizontal, thus providing a 4:3 ration as is common inTV cameras.

An aspect of some embodiments of the invention relates to electrical andmechanical coupling of motors for driving a rotating disc. In someembodiments of the invention, the motor windings of two or more DCmotors are connected in series, such that mechanical mismatch of theirspeeds is compensated for by electrical voltage redistribution betweenthe motors, without requiring any additional circuitry. Thus, bothmotors engage the disc equally, substantially without any slippage, evenif there is some difference in radius between the wheels coupled to themotors and/or the disc is uneven. Another possible advantage is theability to use lower quality motors whose design characteristics are nottightly controlled, without the adverse effect of added noise and/orvibration.

An aspect of some embodiments of the invention relates to a calibrationstep for a rotating disc lens protection device. In some embodiments ofthe invention, the disc is mounted on a small plurality of wheels.During a calibration step, the disc is rotated and the eccentricity ofthe wheels relative to their axes is corrected, thus reducing wear,vibration and/or noise.

An aspect of some embodiments of the invention relates to a transparentdisc being easily removable for cleaning without taking apart the lensprotector. In some embodiments of the invention, the transparent disccan be snapped off the engaging wheels, once a rim cover is removed.

An aspect of some embodiments of the invention relate to a method ofconnecting wheels for gripping a transparent disc, on a motor forrotating the wheels. In some embodiments of the invention, the wheelsare supported by a case, which may also support the motor. The axis ofthe motor is coupled to one or more wheels, via a plurality of dampingpins, such as rubber pins. In some embodiments of the invention, aplastic molding is provided on the axis and this plastic molding ispinned to the wheels. In some embodiments of the invention, by notsupporting the disc on the motor, vibration coupling between the motorand the disc are reduced by the indirect connection between them.

There is thus provided in accordance with some some embodiments of theinvention, a lens protecting mechanism, comprising:

a transparent disc having a rim;

a plurality of wheels that engage and support the rim; and

at least one motor, coupled to at least one of said wheels to applyrotational force to said disc.

Optionally, said plurality of wheels comprises wheels havingsubstantially identical mechanical properties. Alternatively oradditionally, said plurality of wheels comprises four wheels.Alternatively or additionally, said al least one motor is coupled tosaid wheel using a rubber pin and wherein said motor does notsubstantially support said wheel against said disc. Alternatively oradditionally, said wheels and said disc are constructed such that theangular inertia of the wheels substantially balances out the angularinertia of the disc. Alternatively or additionally, said wheels arearranged at substantially ±45° to expected movement axis of a lens forwhich lens protection mechanism is adapted.

There is also provided in accordance with some some embodiments of theinvention, a lens protection mechanism, comprising:

a rotating transparent disc; and

at least one counter-rotating object whose rotation substantiallycancels out angular inertia caused by the rotation of the disc.

Optionally, said at least one counter-rotating object supports saiddisc. Alternatively or additionally, said at least one counter-rotatingobject applies rotational force to said disc.

There is also provided in accordance with some embodiments of theinvention, apparatus for applying force to a rotating disc, comprising:

a first DC motor coupled to a first wheel for engaging said disc; and

a second DC motor coupled to a second wheel for simultaneously engagingsaid disc; and

wiring that connects said DC motors in series to a power source, suchthat the voltage distribution between the motors reflects theinstantaneous rotational speed of each motor.

There is also provided in accordance with some embodiments of theinvention, lens protection apparatus, comprising:

a rotating transparent disc;

at least one wheel, coupled to said disc and causing said disc torotate; and

a rim on said disc snap-connecting said disc to said apparatus, suchthat said disc can be snapped off said apparatus by applying pressure tothe disc. Optionally, said rim is snap-connected onto said wheel.Alternatively or additionally, said rim is resilient.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention are described belowwith reference to figures attached hereto. In the figures, identicalstructures, elements or parts that appear in more than one figure aregenerally labeled with the same or similar number in all the figures inwhich they appear. Dimensions of components and features shown in thefigures are chosen for convenience and clarity of presentation and arenot necessarily shown to scale. The figures are listed below:

FIG. 1A is a side view of a lens protector mounted on a camera, inaccordance with some embodiments of the invention;

FIG. 1B is a front view of a lens protector, in accordance with someembodiments of the invention;

FIG. 2 is a perspective view of a main subassembly of the lens protectorof FIG. 1, showing the moving parts of the protector, in accordance withsome embodiments of the invention;

FIG. 3 is a cross-sectional view of a motorized wheel, in accordancewith some some embodiments of the invention;

FIG. 4 is a cross-sectional view of a free-turning wheel, in accordancewith some some embodiments of the invention;

FIG. 5 is a cross-sectional view of a ring for coupling a disc to thewheels of FIGS. 3 and 4, in accordance with some embodiments of theinvention; and

FIG. 6 is a cross-sectional view through the lens protector of FIG. 1B,cutting across one motorized wheel, through a transparent disc and afree-turning wheel in accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

FIG. 1A is a side view of a configuration 100 comprising a lensprotector 104 mounted on a camera 102, in accordance with someembodiments of the invention. A typical, but not necessarily limitingapplication of a lens protector is for mounting on a camera. As shown inFIG. 1A, a typical camera 102 comprises a body 106, a battery 108, ashoulder grip (or hand grip) 110 and a lens 112. In a manual orautomatic focus camera, a focus (and/or zoom) ring control 114 isoptionally provided.

As shown, lens protector 104 is placed in front of lens section 112, andis optionally coupled by a coupler 116 directly to body 106 of thecamera. An optional control box 120, which can include a power source,electronic and/or manual controls, which may be mounted on protector104, is shown as well. Optionally, a flared rim 118 is provided to guideremoved water and debris away from the camera. Not shown are a pluralityof optional openings in the bottom of protector 104, for draining ofwater that may enter the protector.

FIG. 1B is a front view of lens protector 104, in accordance with someembodiments of the invention. A disc 122 is centrally located insideflared rim 118. A housing 124 is shown enclosing the mechanism ofprotector 104 (described below). Disc 122 is optionally made ofoptically transparent glass. As used herein, “transparent” means:allowing the passing through of sufficient energy of the relevantwavelengths of electromagnetic radiation for forming an image by thecamera. In some embodiments transparent disc 122 may be coated, forexample to reduce reflection. Disc 122 may have filtering functionality,for example a “grey”, a UV, a polarizing, or a “neutral density” filter.In some embodiments disc 122 may be a lens.

Alternatively or additionally, in some embodiments, protector 104 isintegrated with lens section 112 or camera 102, rather than being aseparate device.

FIG. 2 is a perspective view of a main subassembly 140 of lens protector104, showing the moving parts of protector 104, in accordance with someembodiments of the invention. A rigid body element 142, which is coupledto housing 124, optionally has mounted thereon a plurality of freeturning wheels 144 and a plurality of motorized wheels 146, powered by aplurality of motors 148. In some embodiments of the invention, bodyelement 142 is attached to housing 124 via a plurality, such as four, ofrubber pins, for example pins having a shear strength of 55 and adiameter of 6 mm. Body element 142 is optionally made of metal, howeverother materials, such as plastic and hard rubber may also be used.

A ring 150, is engaged by wheels 144 and 146 and couples between thewheels and transparent disc 122. Optionally, transparent disc 122 ismounted on ring 150. Alternatively, ring 150 may be an outer edge ofdisc 122.

In some embodiments of the invention, wheels 146 supply power to rotatering 150 and disc 122, while wheels 144 act as bearings for therotational motion.

Several problems, including noise and reduced image quality can becaused by the vibration of protector 104 and/or the coupling of suchvibration to camera 102.

In some embodiments of the invention, the arrangement of wheels, thedesign of the wheels and/or the mechanics of the wheel coupling to body142 is symmetrical or asymmetric around ring 150. This, and/or otherfeatures described below are optionally provided for reducing thevibration of protector 104. It is noted that by using same elements (thewheels) to hold, balance and rotate disc 122, proper design of thewheels and their balancing can be more easily achieved than if separateelements are used for each function. However, this is not an essentialfeature of the invention, albeit a desirable one. It is noted that inthe design shown in FIG. 3 and FIG. 4, both motorized and free-tuningwheels provided exactly the same support to disc 122 and differ only inthat the motorized wheels also provide power.

What is shown in FIG. 2 is a two way symmetry (around vertical andhorizontal axes), however complete symmetry (also rotational) may alsobe provided. Alternatively or additionally to providing four wheels, adifferent number of wheels, for example three may be provided. Thedesign shown is optionally used so that the wheels are located atexpected directions of application of gyroscopic force. This force isexpected to act at about 45° to the main movement axes, which incameras, are usually parallel and perpendicular to the gravitationalfield and disc 122.

Alternatively or additionally to vibration coupling problems, gyroscopicinertia of the protector may also be a problem, especially if arotational speed of 2000-3000 RPM, or higher, such as 3000-4000 RPM isprovided. It is expected that various features described herein may beused in the design of lens protectors with higher rotational speedsand/or larger sizes, utilizing the advantages that may be achieved, forexample as described herein.

In some embodiments of the invention, the protector is designed so thatthe angular moment of the moving parts substantially cancels itself out.To that effect, the weight (and radial distribution of the weight) ofthe wheels is matched to the weight of the disc, taking into account therelative radii of the wheels and disc. Thus, as the wheels turn inopposite direction to the disc, the angular moments of the wheel scancancel out those of the disc, even when the rotation is started. In anexemplary embodiment, the disc weighs 17 grams and the wheels, each,weigh 28 grams. Alternatively, the disc angular moment is canceled outby a second disc having a similar radius and rotating in an oppositedirection.

FIG. 3 is a cross-sectional view of an exemplary motorized wheel 146, inaccordance with some embodiments of the invention.

FIG. 4 is a cross-sectional view of an exemplary free-turning wheel 144,in accordance with some embodiments of the invention.

Although the motorized wheels 146 and the free-turning wheels 144 candiffer in several respects, as described below, in some embodiments ofthe invention, a substantially same wheel rim 162 is provided in bothwheel types for engaging ring 150. Alternatively or additionally, asubstantially same wheel body 160 optionally is provided in both wheeltypes. In some embodiments of the invention, the wheels are coupled tobody plate 142 in a same manner, by a casing, on which the wheels areaxially mounted. The casing optionally includes a flange so that thecasing can be coupled to body plate 142 by a nut the engages the bodyplate between the nut and the flange. The casing may be eccentric, forexample for the free-turning wheels, allowing correction of the distancebetween the wheel centers.

In some embodiments of the invention, wheel rim 162 is formed of a lowdistortion and compression material, which also optionally has suitablefriction characteristics for engaging ring 150, for examplethermo-rubber. In some embodiments of the invention, wheel rim 162 isrequired to be precision made, and to maintain its dimensions over anormally encountered range of humidity and temperature, for examplebetween −40° C. and +50° C. and between 1% and 100% relative humidity.In some embodiments of the invention, wheel rim 162 is formed of a castthermo-rubber that is also selected to have a high damping factor, so asto reduce resonance effects.

In an exemplary embodiment, the above rubber is first molded (withvulcanizing) on the wheels, then machined and then ground to the exactdesired shape. These processes are typically required due to therelatively low tolerances achieved with standard rubber manufacturingtechniques, while it is desirable to have very precise machining, forexample no more than 4 micron run off, although 10 microns run-off mayalso be acceptable.

Various considerations may be taken into account during manufacturing,to reduce distortion caused by the manufacturing processes. In oneexample distortions of the wheels caused by molding the rubber may beprevented by molding the wheel rims while the wheel is not completelymachined (using CNC) and then completing the machining after themolding, and optionally, before the grinding of the wheel rim.

As illustrated in FIG. 3 a plastic molding 156 is optionally attached toan axis 154 of the motor. Optionally, molding 156 is coupled to wheel160 using rubber pins, one pin 158 being shown. Optionally, three pinsare used. A casing 151 is optionally provided, which grasps motor 148and optionally surrounds axis 154. In a space 164 formed between casing151 and wheel 160, a plurality of ball bearings are optionally provided.Wheel 160 is optionally locked onto (and supported by) casing 151 (andoptionally not axis 154) by a locking ring. Casing 151 is optionallycoupled to body 142 by compression, between a flange of casing 151 and alocking nut 152. In some embodiments of the invention, casing 151 ismovable in the plane of body 142, for example casing 151 beingeccentric, so that axis 154 can be moved relative to disc 122.Alternatively to rubber pins 158, a slip-contact may be provided betweenwheel 160 and plastic molding 156.

FIG. 4 illustrates a free turning wheel 144, in which a wheel 160 ismounted on a hinge 174, formed of a casing 170. A locking ring 176 isoptionally provided to lock the wheel on the axis. A spacing 178 formedbetween wheel 160 and hinge 174 optionally contains ball bearings.Casing 170 is optionally locked to body 142 in a same manner as iscasing 151 (FIG. 3), using compression between a flange and a lockingnut 172. Alternatively or additionally to motorized wheels, some of thefree-turning wheels may include means for moving casing 170 relative tobody 142.

FIG. 5 is a cross-sectional view of ring 150, used for coupling disc 122to wheels 144 and 146, in accordance with some embodiments of theinvention. As shown, disc 122 is held between a back lip 182 and a frontlip 180. Wheel rim 162 is held between two side rims 186. In someembodiments of the invention, an inclined portion 184, is provided, tobe in contact with the edge of wheel rim 162, to stabilize disc 122relative to wheel rim 162. In an exemplary embodiment of the invention,the lips 180 and 182 of disc-holder 150 are at an oblique angle to thesurface of disc 122. Alternatively or additionally, the angles 183between portions 184 and portions 180 and 182 are rounded. Alternativelyor additionally, the edge 185 of portion 186 is rounded. Alternativelyor additionally, edge 187 of portion 186 is rounded, or alternativelyhas an straight section starting at the edge and terminating curve whichblends into the inclined surface of portion 184.

In some embodiments of the invention, ring 150 is made of Delarin andhas a run out tolerance of 20 micron. In some embodiments of theinvention, ring 150 is heated, for example to 50° C. or 70° C. beforebeing pushed into the space between lips 182 and 180, thus expandingring 150 and allowing easier insertion of disc 122. Once disc 122 isinserted, it optionally straightens out the ring. In some embodiments ofthe invention, the use of a resilient plastic, and especially of theresilience of rims 186, allows the disc to be snapped off of the wheels,without taking the entire lens protector apart. Alternatively oradditionally, ring 150 absorbs vibration.

Disc 122 is optionally formed of plate glass and ground to have a radialaccuracy of 20 microns and a thickness variation of less than 1%.However, these numbers may vary depending on the application.

FIG. 6 is a cross-sectional view through lens protector 104, cuttingacross one motorized wheel 146, through the center of disc 122 and afree-turning wheel 144. As can be seen the casing surrounding themotorized wheel is generally larger than the casing surrounding thefree-turning wheel. Optionally, the attachment to camera 102 is at theside of the motorized wheels. Alternatively, the attachment is betweenthe bottom of the lens protector and camera body 106. In someembodiments of the invention, the coupling is directly to camera body106, to reduce the transmission of vibrations to the lens of camera 102.

In some embodiments of the invention, the motors are powered by thebattery of the camera, for example being fed off of an existing powersocket in the camera (e.g, a projector lamp socket). Alternatively, themotors are connected directly to the camera power supply. Alternatively,a battery, in some embodiments rechargeable, is provided integral withlens protector 104, optionally inside of housing 124, for example in box120 (FIG. 1).

In some embodiments of the invention, the two or more DC motors areelectrically connected in series, in addition to them being mechanicallycoupled via ring 150. This interconnection allows the motors todistribute the available voltage between themselves, without requiringan external control circuit. For example, if one wheel has a smallerradius than the other wheel the motor coupled to the larger wheel needs(and will receive) a lower voltage than the smaller wheel and slippagewill be prevented. This distribution of voltage to prevent slippage (andassociated wear, noise and vibration) optimally compensates fordifferences in the wheel radius without requiring exact calibration ofmotor rotation speeds. Additionally, this redistribution may alsocompensate for variations in wheel and disc diameters.

As noted above, an on/off switch may be provided on control box 120.Protector 104 may be configured in several ways. It may be an automaticdevice that is on at all times. Alternatively, it may be manuallyswitched on and off, using said switch. Alternatively, protector 104 maybe automatic, for example switching on when any debris or water isdetected on disc 122. In some embodiments of the invention, the humidityinside or outside protector 104 is monitored. If the humidity approaches100%, it is assumed that wet conditions apply and lens protector 104 isactivated.

In some embodiments of the invention, the switch is located on lensprotector 104, adjacent where the camera operator's fingers are locatedduring operation of the camera. Alternatively, the switch is connectedto lens protector 104 by a wire and is located on camera body 106 oreven on lens section 112. Optionally, the switch is adhesive and can beattached to any desired location on the camera. Possibly, a wirelessswitch is provided, alleviating the need to keep the wire out of theway.

The above lens protector has been described mainly for hand-heldcameras. However, such protection may also be provided for other typesof cameras, for example industrial cameras and surveillance cameras.Optionally, detection of debris or water on disc 122 is by imageprocessing the image acquired by the camera, for example by detectingreduction in focus quality. Alternatively or additionally, the disc maybe slowly rotated and changes in the image correlated with the discrotation can be used to determine the presence of debris or water. Theuse of image processing is especially indicated in setting where theimage is automatically processed for other reasons, such assurveillance.

It should be appreciated that in some application, for exampleindustrial imaging, noise is less of a problems, so tolerances,especially of run-outs may be poorer. Also, there may be fewerrestrictions of the weight of protector 104. For example, noise can bereduced by making disc 122 thicker, since a thin disc acts as a membranefor the acoustic vibrations in protector 104. However, such a thickeningwill increase the weight of protector 104, as also the wheel will needto be increased substantially in weight.

It is generally expected that the run-out tolerances affect the noise,but do not substantially affect the gyroscopic properties of the lensprotector. In an exemplary reduced quality application, a wheel run-outof 30 microns and a disc/ring run-out of 100 micron, will still operate,albeit with increased noise and possibly wear and vibration.

In one manufacturing method, the parts are first cast or injected andthen machined using CNC and then a quality check is made to reject partsthat are out of tolerance. Depending on the desired quality, one or moreof the quality enhancing steps may be omitted or reduced in strictness.It is however noted that by using poorer tolerances, a reduction in lifetime of the lens protector may also result.

Optionally, a thermostat may be provided, for heating disc 122, forremoving condensation on disc 122 and on lens 112. Disc 122 may beheated, for example, using radiation heaters, inside of housing 124.Alternatively, spinning disc 122 will remove such condensation.

In a medical application, a lens protector is provided for protectingendoscope lens inside the body. In an exemplary application, the outerdiameter is between 10 and 20 mm, however, smaller devices, such as 5 mmor smaller may also be provided. Generally, the manufacturing will besimpler, as noise and/or longevity may not be important considerationand tolerances may be easier to achieve as the parts have a smaller selfweight.

It will be appreciated that the above described methods of protectinglens and the various apparatus described may be varied in many ways. Inaddition, a multiplicity of various features, both of methods and ofdevices have been described. It should be appreciated that differentfeatures may be combined in different ways. In particular, not all thefeatures shown above in a particular embodiment are necessary in everysimilar embodiment of the invention. Further, combinations of the abovefeatures are also considered to be within the scope of some embodimentsof the invention. When used in the following claims, the terms“comprises”, “includes”, “have” and their conjugates mean “including butnot limited to”.

What is claimed is:
 1. A lens protecting mechanism, comprising: atransparent disc having a rim; a plurality of wheels that engage andsupport the rim; and at least one motor, coupled to at least one of saidwheels to apply rotational force to said disc.
 2. A mechanism accordingto claim 1, wherein said plurality of wheels comprises wheels havingsubstantially identical mechanical properties.
 3. A mechanism accordingto claim 2, wherein said wheels and said disc are constructed such thatthe angular inertia of the wheels substantially balances out the angularinertia of the disc.
 4. A mechanism according to claim 1, wherein saidplurality of wheels comprises four wheels.
 5. A mechanism according toclaim 4, wherein said wheels and said disc are constructed such that theangular inertia of the wheels substantially balances out the angularinertia of the disc.
 6. A mechanism according to claim 1, wherein saidat least one motor is coupled to said wheel using a rubber pin andwherein said at least one motor does not substantially support saidwheel against said disc.
 7. A mechanism according to claim 6, whereinsaid wheels and said disc are constructed such that the angular inertiaof the wheels substantially balances out the angular inertia of thedisc.
 8. A mechanism according to claim 1, wherein said wheels and saiddisc are constructed such that the angular inertia of the wheelssubstantially balances out the angular inertia of the disc.
 9. Amechanism according to claim 1, wherein said wheels are arranged atsubstantially ±45° to expected movement axes of a lens for which thelens protection mechanism is adapted.
 10. A mechanism according to claim1, further comprising: at least one counter rotating wheel whoserotation substantially cancels out angular momentum caused by therotation of the disc.
 11. A mechanism according to claim 10, whereinsaid at least one counter rotating wheel is connected to said motor. 12.A mechanism according to claim 1, wherein said at least one motor is aDC motor; and further comprising: a second DC motor coupled to a secondwheel for simultaneously engaging said disc; and wiring that connectssaid DC motors in series to a power source, such that the voltagedistribution between the motors reflects the instantaneous rotationalspeed of each motor.
 13. A mechanism according to claim 1, wherein saidrim is removably snap-connecting said disc to said engaging wheel, suchthat said disc can be snapped off said engaging wheel by applying forceto the disc.
 14. Apparatus according to claim 13, wherein said rim issnap-connected onto said wheel.
 15. Apparatus according to claim 13,wherein said rim is resilient.
 16. Apparatus according to claim 1, inwhich the some or all of the angles and edges of said rim coupling ofthe motor to said rotating transparent disc are rounded.